This invention relates to filters of microwave electromagnetic signals having at least one resonant chamber and, more particularly, to a microwave filter wherein each resonant chamber is a cavity partially loaded with a dielectric block and having coupling slots located away from the dielectric block to provide for a concentration of higher-order modes of electromagnetic waves in the block away from the coupling slots for attenuating higher-order modes outside a passband of the filter.
Microwave filters are employed frequently for processing signals. For example, a band pass microwave filter may be coupled to an antenna for reception of an incoming electromagnetic signal, the band pass filter passing signal components lying within a desired frequency band while excluding signal components lying outside of the band. A common form of construction of such filters is the use of a series of cavities formed within a metallic structure to serve as resonators, the hollow cavities being coupled via slots in walls which separate the cavities from each other.
A problem arises in the utilization of such filter structures in that electromagnetic waves introduced into such filter structures propagate by both a fundamental mode and higher-order modes. Due to the propagation of the higher-order modes, a filter having a stop band at frequencies immediately above the passband experience a transmission of signal components at still higher frequencies, which higher frequencies correspond to the frequencies of the higher-order modes. Therefore, it is a common practice in the design of microwave filters to include a further filter structure providing the function of a low pass filter having a cut-off frequency greater than the frequencies of the passband but lower than the frequencies of the higher-order modes.
While the use of the additional low pass filter structure is effective in terms of accomplishing the desired band pass filter function, the additional filter structure has a physical complexity and additional physical size to the filter assembly which is disadvantageous in microwave systems requiring a minimum overall size, such as a microwave system to be carried by a satellite. Also, the additional low pass filter may not provide as sharp cut-off frequency response as is desired for construction of an enlarged passband of the band pass filter. As a result, existing filter technology is restrictive in the available bandwidth and also necessitates undesirably large physical size, particularly for airborne and satellite communication systems.